Background on patent application for a gas sampling system utilizing a remote ceramic gas tube assembly and ceramic oxygen probe combination to be used for analyzing the oxygen partial pressure, Po.sub.2, in a gas carburizing atmosphere.
Oxygen probes utilizing a solid electrolyte have been used to control the carbon potential in carburizing atmosphere for the last fifteen years. The oxygen probes are typically installed directly into the furnace atmosphere (i.e. an in situ type installation). A discussion of carbon potential control in carburizing is described in Control of Surface Carbon Content, Metals Handbook, Vol. 4, pp. 417-431, 9th Edition, 1981. When an in situ type oxygen probe is used to measure the oxygen content in an atmosphere used for carburizing or neutral hardening, the general guideline is that the probe should be exposed to the same gas atmosphere and temperature as the work is exposed to. A frequent problem in some installations is that the length of the probe is too short to meet the above general guidelines for an in situ type probe installation. For example, in most rotary retort type carburizing furnaces, the length of a typical commercial oxygen probe used for controlling the carburizing process is not long enough to reach that part of the furnace which has the same atmosphere and temperature that the parts being carburized are exposed to. Another example of this problem is an endothermic generator where natural gas and air are reacted at an elevated temperature (e.g. 1900.degree. F.) in the presence of a nickel catalyst to produce an endothermic atmosphere. For a description of endothermic generators, see for example: Metals Handbook, Vol 4, pp. 389-416, 9th Edition, 1981. The use of an in situ type oxygen probe in some commercial endothermic generators is not feasible because either the length of the probe is insufficient or because of the difficulties and cost associated with this type of installation.
An alternative to the in situ type oxygen probe analysis is to use an oxygen probe in conjunction with a gas sampling technique. The Davis U.S. Pat. Nos. 3,058,815 and 3,011,873 show the use of a separate furnace to maintain a gas measuring sensor at a temperature comparable to that of the gas in the heat treating furnace.
The following is a brief discussion of problems that arise when an oxygen probe is used in conjunction with a gas sampling technique to analyze a gas atmosphere removed from either an endothermic generator or a carburizing furnace. The composition of a typical carrier or carburizing atmosphere is described in Metals Handbook, Vol. 4, pp. 417-431, 9th Edition, 1981. When an endothermic or carburizing atmosphere is cooled by removing it from a heated chamber, the following reactions take place: EQU CO+H.sub.2 C(soot)+H.sub.2 O, EQU 2CO C(soot)+CO.sub.2
The reason that the carburizing atmosphere tends to produce carbon in the form of soot as the temperature is lowered is that the equilibrium constant for both of the above reactions increases with decreasing temperature.
Thus, cooling of an atmosphere with a carbon potential will favor the formation of carbon (soot) and increase the concentration of H.sub.2 O and CO.sub.2 in the gas sampled atmosphere. The amount of H.sub.2 O, CO.sub.2 and carbon (soot) formed by the above reactions will depend on the kinetics (i.e. these reactions are time and temperature dependent). If catalytic surfaces are present, the rate at which these reactions proceed will be increased. It should be noted that according to thermodynamic considerations the formation of carbon (soot) and the equilibrium partial pressure of H.sub.2 O and CO.sub.2 increase as the temperature decreases. The rate of these reactions, however, will decrease as the temperature is decreased according to kinetic theory. Thus, there is a temperature range where the formation of carbon (soot) and increased concentration of H.sub.2 O and CO.sub.2 will be favored (i.e. approximately 900.degree. F.-1500.degree. F.). The lower temperature limit is determined primarily by kinetic considerations. Whereas, the upper temperature limit is determined by both thermodynamic and kinetic considerations and the composition of the carburizing atmosphere. For example, if an atmosphere with a high carbon potential is used, the upper temperature limit where sooting may begin is increased and if a gas with a low carbon potential is used the upper temperature limit is decreased. Similar problems are encountered when the sampled gas is reheated in the sampled gas tube assembly. During reheating, the temperature of the carrier or carburizing gas increases until the temperature is the same as the endothermic generator or furnace in which it is placed. During the time that the temperature of the gas is reheating, between approximately 900.degree. F.-1500.degree. F., sooting will tend to occur as described in the aforementioned reactions. In the current state of the art, the gas is reheated in a heat-resistant, stainless steel alloy tube. A problem in practice is that the metal alloy tube fills up with soot which either blocks the flow of gas or effects the accuracy of the oxygen probe analysis. The soot then has to be removed by physically taking the sampled gas tube apart and cleaning it. The frequency of cleaning the metal alloy tube may be as often as every few days which is very undesirable. One of the problems with the current state of the art is that the metal alloy tube used to reheat the carburizing atmosphere catalyzes the above sooting reactions. Because of the sooting reaction the composition of the atmosphere is altered. For example, both the CO.sub.2 and H.sub.2 O content is increased. Thus, the voltage output of the oxygen probe is also altered resulting in an inaccurate gas analysis.